At issue are the diagnostic guidelines used to determine what constitutes a “mental disorder.” These are delineated by the American Psychiatric Association and codified in the Diagnostic and Statistical Manual of Mental Disorders. (The DSM-V is scheduled to appear in 2013.) As someone who has been directly involved in updating the manual, the author struggles with a thorny question: “Does our system of diagnosing mental disorders fail to distinguish normal human suffering from genuine mental illness? Or are we really getting sicker?” Since the 1980 publication of DSM-III, critics have accused the profession of being self-interested, “expanding the boundaries of mental disorders” to “relatively trivial problems” such as “caffeine-induced sleep disorder” or blurring the difference between shyness and “social phobia.” Yet people do seek psychological help in alleviating suffering, and eligibility for health-insurance coverage is dependent on the diagnosis they receive. While pharmaceutical companies may be charged with encouraging broad definitions of mental illness, the opposite is true of insurers. McNally reviews advances in the field since the publication of DSM-III, examining research in evolutionary psychology, the role of social norms in defining maladaptive behavior and the interplay of genetics and environment. He also points to how nonpsychiatric medicine has shifted the “boundary between health and sickness” by treating people with preventative medication for high blood sugar, blood pressure and cholesterol. Whether or not—and how—to treat psychological difficulties will remain a problem that individuals, medical professionals and society at large will grapple with, but McNally is optimistic.